Oh yeah, the shit about his parents and Solidus I knew. From MGS2, I think? The shit just seems to pile on, keep in mind I'm only 2/5 of the way through MGS4 yet (starting act 3 now) and from what I hear there's more to be told about Raiden yet. :P Honestly all this make me want to play 2 again.
As far as MGRR goes, well, we all know he's out fore revenge. And Rose called him unstable (I would be too if I grew up like that, just saying).
Related: He just showed up and fucked up a bunch of Gekkos. I like this game. Also: Jennifer Hale.